In the optoelectronics art, it has become increasingly popular to mount a plurality of optical components on a silicon substrate, a technology also known as silicon optical bench technology. (See, e.g., U.S. Provisional Patent Application of Anigbo, Ser. No. 60/009116, filed Dec. 22, 1995 which has been converted to patent application Ser. No. 08/764960.) Such assemblies typically include a semiconductor laser mounted on the surface of the silicon substrate in alignment with a light guiding element such as a spherical lens which is mounted within a cavity formed in the silicon substrate. The lens is typically made of a high index material such as yttrium aluminum garnet (YAG) or spinnel. The lens can be bonded into the cavity in a variety of means such as epoxy, solder or aluminum oxide bonding (AlO bonding). A generally preferred method is AlO bonding, since it forms an inorganic bond, and therefore no organic material is present after bonding. (See, e.g., U.S. Pat. No. 5,178,319 issued to Coucoulas.)
One of the problems encountered with fabricating an AlO based assembly results from the fact that the aluminum bonding layer is typically formed by means of a lift off patterning technique using a photoresist mask covering the laser bonding pad which is usually a multilayer of titanium/platinum/gold. This is done to prevent the formation of gold/Aluminum intermetallic. When the photoresist is stripped off a galvanic potential develops between the Ti/Pt/Au and the aluminum, which can lead to anodization (i.e., corrosion) of the aluminum. This corrosion product acts to reduce the bond strength of the Alo bonded lens.
It is, therefore, desirable to prevent the corrosion of aluminum in the fabrication of assemblies which employ both aluminum and Ti/Pt/Au on the same substrate.